For a game that likes to promote (If not downright force a player to) using a compositional army, Terran sure doesn't have to follow that. Unless you consider 2 Offensive units, and 1 support caster a composition.
I feel that right now MMM is gamebreaking. Whenever I random T and I am up against a Z or P I know it's going to be an easy game, and it almost always is. Unless the Zerg can get off some lucky baneling hits he's going to die. Unless the Protoss can keep me contained or out of his base long enough with forcefield, to get colossi he's going to die. Even then I could be producing tanks or a few vikings to counter them. Storm is laughable with proper micro it doesn't give you that much trouble. I just feel that MMM is too strong right now. The other races have to work so much harder to beat your much smaller army.
2. Easy and Good Macro System
Now due to the fact that Terran starts off slower than the other races due to only being able to produce 1 worker at a time, I feel this is somewhat okay, but it's still a little too good if you ask me. Because of these little guys...
Mules are really great mineral gatherers. Once you get up an orbital command and start pumping these guys you pretty much don't have to worry about your mineral resources anymore. I would really find this system fine if the other races didn't have such less forgiving methods; mainly Zerg.
With Zerg you have spawn larva, which takes a very long time to complete. If you miss casting it when you can that is just wasted time, and production. It really doesn't matter how much energy your queen has saved up.
With Protoss you can chronoboost your buildings. Now I actually feel this is balanced somewhat due to the fact you can chronoboost everything. I still don't feel it's as good as Terran's ability to gain such a huge economic lead though. Sure you may able to spit out a million probes, but they can only collect so fast. With Mules they can overlap workers on mineral patches, and they don't take any supply.
It's just not the minerals that are an issue either. If a Terran forgets to spend his energy it saves up to 200 which is enormous. This allows him to throw down multiple mules, or even increase his supply. Terran isn't disadvantaged by player error very badly compared to P/Z.
3. Terran Controls the Game
Due to MMM being very strong a Terran can gain control of the game very early. They can easily contain Z or P, and they're free to do whatever they want. The other races have to react to your actions the entire game. It's easily exploited as well. I've won countless games with just MMM use, it didn't even matter what they threw at me. I just feel Terran has it too easy compared to the other two races. With Zerg you're forced to make everything, and the same goes with Protoss. Meanwhile the Terran can just sit in his base and keep making the same 3 units.
With the upcoming nerfs I hope this changes the Terran meta game a bit. This will only matter if they do something about Marauders or Medivacs, which were not mentioned in the incomplete patch notes. So we'll see.